1. Field of the Invention
The present invention relates generally to methods and apparatus for treating breathing and/or cardiac disorders and, more particularly, to methods and apparatus for providing a pressure to an airway of a patient during at least a portion of the breathing cycle to treat obstructive sleep apnea syndrome, chronic obstructive pulmonary disease, congestive heart failure, and other respiratory and/or breathing disorders.
2. Description of the Related Art
During obstructive sleep apnea syndrome (OSAS), the airway is prone to narrowing and/or collapse while the patient sleeps. Continuous positive airway pressure (CPAP) therapy seeks to avoid this narrowing by supplying pressure to splint the airway open. With CPAP, this splinting pressure is constant and is optimized during a sleep study to be sufficient in magnitude to prevent narrowing of the airway. Providing a constant splinting pressure, i.e., CPAP, is a simple solution to the problem posed by the collapsing airway. However, this approach exposes the patient to pressures that are higher than the pressures needed to support the airway for most of the breathing cycle.
During inspiration, the pressure created within the lungs is lower than the pressure at the nose. This pressure difference drives the flow of air into the lungs. This pressure difference creates a pressure gradient in the airway connecting the lungs with the nose. That is to say, the nose is typically at ambient pressure while the lungs and airway of the patient are at sub-ambient or negative pressures. This negative pressure acts upon the airway and contributes to its collapse. CPAP levels are typically set to raise the pressure level in the entire respiratory system to the level required to both eliminate the sub-ambient pressures generated by inspiration and overcome any mechanical collapsing forces that result from the structure of the airway tissues, muscle tone, and body position. The inspiratory pressures , i.e., inspiratory positive airway pressure or xe2x80x9cIPAP,xe2x80x9d in bi-level positive airway pressure systems are set in a similar manner.
During exhalation, a positive pressure gradient exists between the interior of the lungs and the exterior of the body. This positive pressure gradient helps to support the airway during exhalation. At the end of exhalation, the pressure gradient is essentially zero; flow is likewise zero and the airway is unaffected by respiratory efforts. Any collapse of the airway at the end of exhalation is purely a function of the structure of the airway tissues, muscle tone, and body position. Bi-level devices seek to supply the expiratory pressure required to support the airway at the end of exhalation.
It should be noted that over the course of a breathing cycle, the pressure gradients between the lungs and the exterior of the body are not constant. The inspiratory pressure gradient falls from zero at the start of inspiration to a peak negative value and then rises back to zero at the end of inspiration. The expiratory pressure gradient rises from zero at the start of exhalation to a peak value and then falls back to zero as exhalation ends. Because the pressure gradient varies over the breathing cycle, the pressure necessary to overcome airway collapse should ideally vary over the breathing cycle.
Traditional CPAP therapy ignores these variations in pressure requirements and provides therapy at one pressure level. Conventional CPAP is rather crude and offers far from optimal therapy since the CPAP pressure is based solely on a worst-case treatment parameter, i.e., the peak pressure requirements during inspiration.
Representing an advancement over conventional CPAP, bi-level positive airway pressure (bi-level PAP) therapies seek to take advantage of the different pressure requirements to lower the pressure during exhalation. Nevertheless, bi-level therapies also fail to afford optimal treatment because the inspiratory positive airway pressure (IPAP) of bi-level PAP is again based on the patient""s peak needs encountered during inspiration and remains constant over the entire inspiratory phase respiration. Also, during bi-level treatment, the expiratory position airway pressure (EPAP) remains constant and is related solely to the support needs at the end of exhalation.
In addition to OSAS, positive airway pressure therapy, such as bi-level PAP therapy, has been applied in the treatment of other breathing disorders, such as chronic obstructive pulmonary disorder (COPD). One of the problems with this mode of treatment, however, is that the patient has difficulty stopping inspiratory flow. This phenomenon arises due to the disparity between applied IPAP and the pressure needed to overcome the patient""s respiratory resistance at the end of inspiration. As the former pressure typically exceeds the latter, the xe2x80x9csurplusxe2x80x9d IPAP at the end of inspiration leads to uncomfortable and potentially harmful hyperinflation of the patient lungs.
Conversely, in order to begin inspiratory flow, a COPD patient must reduce the pressure inside his lungs to a pressure that is less than the ambient pressure at the inlet of his respiratory system. Due to the condition commonly known as xe2x80x9cAuto-PEEP,xe2x80x9d the pressure in the patient""s lungs is typically above ambient pressure at the end of exhalation. The patient""s breathing muscles thus must perform additional work to expand the lungs and thereby reduce lung pressure below ambient before flow into the lungs can occur. Auto-PEEP is typically treated with a form of resistive counter pressure known as PEEP (positive end expiratory pressure). PEEP is set at a level just below the patient""s Auto-PEEP level, thereby reducing the amount of breathing work required to initiate inspiratory flow.
With conventional treatments, such as pressure support, CPAP or bi-level therapy, PEEP is achieved by applying the same pressure over the entire phase of expiration, e.g., the EPAP phase of bi-level PAP therapy. It should be noted that EPAP is not synonymous with PEEP. EPAP indicates a constant pressure delivered to the patient throughout exhalation, while PEEP indicates positive end expiratory pressure. By definition, the PEEP pressure is only required at the end of exhalation. As such, the administration of EPAP throughout the expiratory cycle to assure that satisfactory PEEP is maintained undesirably contributes to the breathing work that a patient must perform during exhalation.
In addition to CPAP and bi-level PAP, other systems have been proposed for clinical research and/or therapeutic application, including treatment of OSAS, COPD and other breathing disorders, that offer an assortment of methods and apparatus by means of which a subject""s respiratory efforts may be induced, superseded, assisted and/or resisted. Some of these systems perform their prescribed functions responsive to one or more parameters associated with a subject""s respiratory activity including, but not limited to, inspiratory and/or expiratory flow, inspiratory and/or expiratory pressure, tidal volume and symptoms indicative of airway obstruction, e.g., snoring sounds. Some achieve their objectives transthoracically while others deliver air at positive or negative pressure directly to the subject""s airway.
An early example of such a system, commonly referred to as an xe2x80x9ciron lung,xe2x80x9d is disclosed in a publication entitled xe2x80x9cMechanical Assistance to Respiration in Emphysema, Results with a Patient-Controlled Servorespirator,xe2x80x9d authored by James R. Harries, M.D. and John M. Tyler, M.D., published in the American Journal of Medicine, Vol. 36, pp. 68-78, January 1964. The iron lung proposed in that publication is a respirator designed to apply and remove transthoracic pressure to and from the exterior surface of the body of a subject who sits in a large pressurizable chamber in order to assist the patient""s respiratory efforts (i.e., the iron lung applies negative pressure during inspiration and either ambient or positive pressure during expiration). Sophisticated for its day, the apparatus continually controlled the internal chamber pressure in response to the patient""s spontaneous respiration, specifically in response to detected respiratory flow or volume. Indeed, a signal obtained from a strain gauge pneumograph fastened around the patient""s chest was electrically separated into three components: one proportional to volume, another to inspiratory flow and a third to expiratory flow. Each component was assigned a separate gain control. The component signals are then recombined to control the pressure in the chamber by means of an electrically driven variable valve situated between a blower and the chamber.
Although effective for their intended purposes, this and other iron lung devices have generally fallen into disfavor because of their bulk, inconvenience, cost and limited application. That is to say, because of their size and cost such equipment is purchased and maintained essentially exclusively by medical facilities such as hospitals and clinics. Further, iron lungs do not lend themselves to treatment of OSAS and related disorders where comfort and unobtrusiveness are critical for patient compliance and treatment efficacy. This is because negative pressure applied during inspiration compounds the factors that operate to collapse the airway during an inspiratory phase.
An essay entitled, xe2x80x9cAn Apparatus for Altering the Mechanical Load of the Respiratory System,xe2x80x9d authored by M. Younes, D. Bilan, D. Jung and H. Krokes, and published in 1987 by the American Physiological Society, pp. 2491-2499, discloses a system for loading and unloading of a subject""s respiratory efforts to effect various respiratory responses. The system may load or unload during inspiration, expiration, or both, to assist or resist a subject""s spontaneous respiratory activity. The system may apply a continuous positive or negative pressure directly to the subject""s airway and loading or unloading occurs via a command signal generated by detected respiratory flow, volume, applied voltage, an external function, or other source.
A drawback to this system, however, is that a single resistive gain is chosen for resistive loading or unloading. This single gain is applied to a xe2x80x9chalf-wavexe2x80x9d of the respiratory cycle (either inspiration or expiration) or the xe2x80x9cfull-wavexe2x80x9d thereof (both inspiration and expiration). In other words, under full-wave respiratory loading or unloading, a single chosen gain value is employed during both inspiration and expiration. Thus, a gain that may produce favorable results in regard to reducing breathing work during inspiration, for example, may cause less than desirable or even detrimental consequences during expiration. The converse is true for a gain selected specifically for optimizing expiratory work reduction.
In addition, the Younes et al. system operates as a closed, leak-proof system. Hence, to predict its ability to function in an open, leak-tolerant system would be problematic. As such, whether it may be adapted to OSAS treatment, which invariably involves some degree of known and unavoidable unknown system leakage, is suspect.
U.S. Pat. No. 5,107,830 to Younes essentially reiterates all of the xe2x80x9cbreathing assistxe2x80x9d (unloading) disclosure that is covered in the Younes, et al. American Physiological Society publication discussed above. In the system disclosed in U.S. Pat. No. 5,107,830, however, the adjustable pressure gain is only realized during inspiration because pressure output is set to zero during exhalation. Additionally, output pressure is calculated as a function of both detected patient inspiratory flow and volume. Furthermore, the system is applicable to COPD but not OSAS therapy.
An article entitled xe2x80x9cA Device to Provide Respiratory-Mechanical Unloading,xe2x80x9d authored by Chi-sang Poon and Susan A. Ward and published in March 1987 in IEEE Transactions on Biomedical Engineering, Vol. BME-33, No. 3, pp.361-365, is directed to an apparatus which functions somewhat similar to one mode of operation described in both Younes disclosures. That is, the Poon, et al. device may operate to unload a subject""s breathing, but only during inspiration. Poon, et al. provide their inspiratory assistance by establishing a positive mouth pressure throughout inspiration in a constant proportion to instantaneous flow. The constant proportion is achieved by (1) selecting a desired gain for a detected positive mouth pressure signal, (2) calculating the ratio of the gain-modified mouth pressure signal over a detected signal reflecting instantaneous flow, (3) comparing the calculated ratio to a selected reference ratio to generate a valve motor control signal, and (4) using the valve motor control signal to operate a motor that drives a servo valve to control the positive pressure applied to the subject""s airway. Thus, the apparatus output pressure is determined as a function of both detected pressure and flow. Further, the pressure must be output at a value sufficient to maintain a constant ratio of pressure to flow.
A publication entitled xe2x80x9cServo Respirator Constructed from a Positive-Pressure Ventilator,xe2x80x9d by John E. Remmers and Henry Gautier, which was published in August, 1976 in the Journal of Applied Physiology, Vol. 41, No. 2, pp. 252-255, describes a modified ventilator that may function as a xe2x80x9cdemandxe2x80x9d respirator generating a transthoracic pressure proportional to phrenic efferent respiratory discharge. Phrenic efferent respiratory discharge is an indication of the outgoing brain signal to the phrenic nerve, which controls diaphragm function. A phrenic efferent respiratory discharge signal causes the diaphragm to contract whereby the subject exerts an inspiratory effort. The phrenic efferent respiratory discharge serves as the apparatus command signal and is processed to produce a moving time average (MTA) and the subject""s tracheal pressure serves as a negative feedback signal. Like the Poon et al. device, the Remmers et al. apparatus provides respiratory assistance only during inspiration.
An apparatus for automatically regulating the flow and pressure output of a respirator is disclosed in U.S. Pat. No. 3,961,627 to Ernst et al. Like the aforementioned Poon et al. device, however, the Ernst et al. apparatus relies upon an unduly complicated scheme dependent upon detected respiratory pressure and flow in calculating delivered output flow and pressure. More particularly, Ernst et al. propose regulating the delivered flow and pressure of a respiration gas in a respirator during the respiration cycle in which the actual flow and pressure of the respiration gas am measured via a measuring device arranged proximate a patient interface. The measured values are converted into electrical signals and the flow and pressure of the respiration gas are controlled during the inspiration and expiration portions of the respiration cycle via a valve arranged between a respiration gas source and the measuring device. The method for regulating the flow and pressure output comprises (1) measuring the actual flow of respiration gas proximate the patient, (2) measuring the actual pressure of respiration gas proximate the patient, (3) calculating nominal values of flow and pressure from preselected fixed values and the actual values, (4) comparing the actual values measured for the flow and pressure with the nominal values, and (5) obtaining from the comparison a control signal for modulating the valve and thereby regulating the flow and pressure of the respiration gas.
Additionally, apart from its utilization of two detected respiratory parameters (flow and pressure) and the complex manner in which these and other variables are reiteratively processed to produce apparatus flow and pressure output, the Ernst et al. system, although capable of delivering a base pressure equivalent to a patient""s required end expiratory pressure, is nevertheless unable to deliver any pressure less than the base pressure. Consequently, the Ernst et al. apparatus requires the patient to perform more breathing work than is necessary to satisfy his respiratory needs, especially in the expiratory phase of a respiration cycle, thereby deleteriously affecting the patient""s comfort and likelihood of continued compliance with the treatment.
In addition to the treatment of breathing disorders, positive airway pressure therapy has been applied to the treatment of congestive heart failure (CHF). In using CPAP on CHF, the effect of the CPAP is to raise the pressure in the chest cavity surrounding the heart. This has the impact of reducing the amount of pressure the heart has to pump against to move blood into the body. By reducing the pressure the heart works against, the work required of the heart is reduced. This allows the sick heart to rest and potentially to get better.
The pressure in the chest cavity is also impacted by respiration effort. With inspiration, the pressure in the chest is reduced (negative relative to resting pressure) due to inspiratory effort. This forces the heart to pump harder to move blood into the body. With expiration, the pressure in the chest is slightly increased (positive relative to resting pressure) due to the elastic properties of the chest. This allows the heart to decrease its efforts to pump blood. While conventional CPAP can help the heart rest, it has negative aspects for the patient such as increased work of exhalation and discomfort from the pressure.
It is an object of the present invention to provide an uncomplicated system operable to deliver pressurized air to the airway of a patient and readily adaptable to the treatment of OSAS, COPD and other respiratory and/or pulmonary disorders that does not suffer from the disadvantages of conventional pressure application techniques. This object is achieved by providing an apparatus for delivering pressurized breathing gas to an airway of a patient. The apparatus, which is referred to below as a xe2x80x9cproportional positive airway pressurexe2x80x9d or xe2x80x9cPPAPxe2x80x9d apparatus, includes a gas flow generator, a patient interface that couples the gas flow generator to the patient""s airway, a sensor that detects a fluid characteristic associated with a flow of gas within the patient interface, a pressure controller that regulates the pressure of breathing gas provided to the patient, and a control unit that controls the pressure controller.
The control unit controls the pressure controller so that the breathing gas is delivered to the patient at a minimally sufficient pressure during at least a portion of a breathing cycle to perform at least one of the following functions at any given moment: (1) reduce cardiac preload and afterload, in which case the minimally sufficient pressure is a summation of a pressure needed to reduce cardiac preload and afterload in an absence of respiratory loading and a pressure needed to overcome an impact of respiratory loading on cardiac preload and afterload, and (2) prevent airway collapse, in which case the minimally sufficient pressure is a summation of a pressure needed to prevent airway collapse due to mechanical forces resulting from the structures of the patient and a pressure needed to overcome airway collapse due to respiratory effort. The apparatus also includes a selector unit that establishes a first gain. The control unit controls the pressure controller so as to deliver the breathing gas at the minimally sufficient pressure during at least a portion of the breathing cycle based on the first gain and the signal from the sensor.
The PPAP system of the present invention provides airway pressure that is lower than pressures typically necessary to treat OSAS, which is normally treated using conventional CPAP or bi-level PAP therapy. With PPAP, the patient receives exhalation pressures lower than conventional bi-level PAP expiratory positive airway pressure levels and well below conventional CPAP levels. Also, the average pressure delivered during inspiration can be lower than conventional or bi-level PAP inspiratory positive airway pressure or CPAP levels, whereas peak PPAP pressure is roughly equivalent to conventional IPAP or CPAP levels. The PPAP pressure range (peak inspiratory pressure to minimum expiratory pressure) is generally between 2 to 20 cm H2O, with typical values in the 8 to 14 cm H2O range. This is consistent with bi-level PAP therapy where significant comfort/compliance is found with peak inspiratory to minimum expiratory pressure differentials of 6 cm H2O or more. The complexity of titration using the apparatus of the instant invention is roughly equivalent to current bi-level PAP titration. In addition, the titration system may incorporate a feedback circuit to provide fully automated PPAP.
Similar to treatment of OSAS, PPAP also delivers mean airway pressure that is lower than pressures typically necessary to treat COPD using conventional bi-level PAP therapy with PEEP or proportional assist ventilation (PAV) with PEEP. That is, with PPAP, the patient receives average exhalation pressures lower than conventional EPAP levels, average inspiration pressures lower than conventional IPAP, and peak PPAP pressure roughly equivalent to conventional IPAP pressures and conventional peak PAV levels. Hence, less breathing work is required with PPAP than with conventional PAV or bi-level treatments of COPD or OSAS.
It is a further object of the present invention to provide a modified CPAP apparatus that is capable of easily detecting exhalation and modifying the exhalation pressure to match a selected pressure profile. This object is achieved by providing an apparatus that includes a gas flow generator, a patient interface that couples the gas flow generator to the patient""s airway, a sensor that detects a physiological condition that is suitable for use to differentiate between an expiratory phase and an inspiratory phase of a breathing cycle, a pressure controller that regulates the pressure of breathing gas provided to the patient, and a control unit that controls the pressure controller. More specifically, the control unit causes the breathing gas to be delivered at a first pressure level during an inspiratory phase of the breathing cycle, which is consistent with the operation of a conventional CPAP device. However, the control unit causes the breathing gas to be delivered in accordance with a predetermined pressure profile during the expiratory phase of the breathing cycle. This profile provides a decrease in the EPAP provided to the patient. Because the pressure profile can be obtained by controlling the operation of existing CPAP devices, it can be readily implemented on many such devices; thereby providing a better therapy for a patient using existing devices.
It is yet another object of the present invention to provide a system for eliminating oscillations in the flow provided during patient exhalation that can occur with use of the PPAP device. According to a first embodiment of the present invention, this object is achieved by causing the pressure controller to provide a pressure to the patient during expiration that is the greater of (1) a first minimally sufficient pressure that is determined by applying a gain to the signal output by the sensor and (2) a second minimally sufficient pressure that corresponds to a current pressure being provided to the patient. By ensuring that the pressure provided to the patient is always the greater of these two pressures, the pressure received by the patient during expiration does not oscillate, because should the pressure to be provided to the patient begin to decrease below the current pressure, the device will not use the calculated pressure, but will continue to provide the patient with the current pressure, thereby preventing a pressure decrease below the current pressure.
According to a second embodiment of the present invention, the object of preventing oscillations in the patient flow provided during expiration is achieved by causing the pressure controller to provide an expiration pressure that is determined based on a volume of gas to be exhaled and a gain. This gain can be the same gain or a different gain from that applied to the signal from the sensor during inspiration (if any). The volume of gas to be exhaled corresponds to a difference between the current volume of gas in the patient and the volume of gas in the patient at rest.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.